1. Field of the Invention
The present invention relates to hydrophilic membranes containing blends of polyethersulfone and sulfonated polymers or copolymers. In particular, the invention relates to hydrophilic membranes wherein the sulfonated polymers or copolymers are sulfonated before being blended with the polyethersulfone.
2. Background of the Invention
Filtration membranes are useful for numerous applications where it is desirable to purify or separate components of gaseous or liquid mixtures. Some applications include, for example, reverse osmosis, computer chip manufacturing, medical applications, and beverage processing. Because the applications for membranes are diverse and numerous, so too are the structures of the membranes and the materials from which they are made.
Membranes can be classified in various ways. One classification scheme lists types of membranes functionally in increasing order of their size selectivity: gas separation (GS), reverse osmosis (RO), ultrafiltration (UF), and microfiltration (MF).
Membranes can also be classified by their cross-sectional symmetry or asymmetry. A membrane that is symmetric, or isotropic, has relatively constant pore sizes throughout its thickness, while an asymmetric membrane has variable pore sizes, usually having relatively larger pores on one side of the membrane and relatively smaller pores on the opposite side of the membrane. Advances in membrane technology have led to membranes having high degrees of cross-sectional asymmetry, as discussed below in more detail.
The materials of which membranes can be constructed also have a major effect on the applications for which the membranes may be used. For example, one major use for microfiltration membranes is to remove microorganisms such as bacteria and yeast from aqueous solutions in the areas of food technology, medicine, and pharmaceuticals. In these areas, it is important that the membranes withstand sterilization. Membranes are normally sterilized with steam at temperatures greater than 110.degree. C. Many membranes will not withstand exposure to steam sterilization, because the materials from which they are made are not stable in the presence of steam. For example, membranes made of polyacrylonitrile are irreversibly damaged by exposure to steam. Further, other polymers such as polycarbonates and polyamides are hydrolyzed by steam.
Membranes made of materials such as polyetherimides, polysulfones, or polyvinylidene fluoride can be steam sterilized. Membranes made with these materials are hydrophobic, however, and are not spontaneously wettable with water. Water will not pass through a hydrophobic membrane at pressures lower than the bubble point unless the membrane is primed with a fluid. Further, hydrophobic membranes tend to adsorb high molecular weight components and foul.
Sulfone polymers are especially preferred materials for constructing membranes because of their availability, durability, versatility, and amenability to casting conditions that result in a great variety of membrane porosities and structures. As stated earlier, however, membranes prepared from sulfone polymers are hydrophobic. Therefore, in applications requiring operation of membranes in aqueous environments, hydrophobic sulfone membranes may be reacted with, or mixed with, moieties which cause the resulting membranes to become hydrophilic.
For example, hydrophilic membranes have been made by adding hydrophilic compounds such as polyvinylpyrrolidone (PVP) to the membrane. However, the hydrophilic compounds are often water-soluble, and they can leach out during filtration, thereby not only reducing the hydrophilicity of the membrane but also creating a risk of contaminating the filtrate. One can reduce leaching by crosslinking the wetting agent and intertwining it with the membrane polymer. For example, Roesink et al. in U.S. Pat. No. 4,798,847 (now Re. No. 34,296) disclose crosslinking polyvinylpyrrolidone throughout the structure of the polysulfone membranes. However, while crosslinking hydrophilic moieties to membranes minimizes leaching, it can also reduce hydrophilicity in proportion to the number of crosslinks created. Moreover, it adds an additional step and complexity to the formulation and casting process of a membrane.
Hydrophilic, water insoluble polymers have been used to make membranes. Manufacture of membranes containing sulfonated polymers has been disclosed in, for example, U.S. Pat. No. 3,855,122. However, these membranes retain large amounts of salt and are used primarily for reverse osmosis.
Other membranes containing mixtures of sulfonated and non-sulfonated polysulfone are described in U.S. Pat. No. 5,246,582. The membranes are hollow fibers which are suitable for dialysis. They have small pores and are suitable for ultrafiltration, not microfiltration. The membranes are therefore limited in their application. In particular, microfiltration membranes are required for most applications in the food, medicine, and pharmaceutical industries.
Further, the only sulfonated polysulfone which was used in the '582 patent was sulfonated polyethersulfone. Expanding the range of sulfonated raw materials to be blended into the membranes could lead to a broader range of properties. Finally, those membranes are isotropic, as evidenced by the micrographs in the '582 patent. Asymmetric membranes have advantages over isotropic membranes, such as higher capacity.
Hydrophilic membranes containing sulfonated polymers have also been made by sulfonating polysulfone membranes. See, for example, U.S. Pat. No. 4,866,099. Sulfonating already prepared membranes generates defects, and the resulting membranes have low flow rates, inconsistent wettability, wrinkles, and low solute retention.
Composite membranes containing polysulfone and sulfonated polymers have been disclosed in, for example, U.S. Pat. No. 5,693,740. The composite membranes comprise a thin film of the polymer supported on an ultrafiltration membrane. Forming composite membranes requires two process steps, forming the support membrane and adding the film. Composite membranes are therefore more expensive and require more complicated manufacture than membranes that can be formed in a single step.
Another challenge in the manufacture of membranes is the creation of a membrane with pores that are small enough to retain macromolecules, while maintaining an acceptable flow rate of the fluid to be filtered. A membrane's resistance to fluid flow is a function of the diameter of the smallest, or retentive, pores through which the fluid must pass, and is also a function of the thickness of the layer of retentive pores.
Some filtration membranes have a layer of very small pores (termed herein a "skin") on one side, while other membranes do not contain this type of layer (termed herein "skinless"). The asymmetry of the pores within the membrane can vary, depending on the conditions under which the membrane is produced. For example, a perfectly symmetrical membrane would have pores of the same diameter on both faces and throughout the support structure between the two faces. However, a highly asymmetric membrane may have pores that change in diameter by 10:1, 100:1, 1,000:1, 10,000:1 or more from one face to the other. Asymmetric membranes are useful in many applications. For example, such membranes can be used for a variety of filtration applications for purification and testing in the food and beverage industry, water treatment, pharmaceuticals, and in medical laboratories.
There are advantages to both symmetric and asymmetric membranes. In general, however, asymmetric membranes are preferred, because the wide pores act as a prefilter to retain particles that are much larger than the skin pores before they come into contact with the skin layer. The prefiltering effect reduces plugging and prolongs the lifetime of the membrane.
Asymmetric membranes are well known in the art. For example, Wrasidlo in U.S. Pat. Nos. 4,629,563 and 4,774,039 and Zepf in U.S. Pat. Nos. 5,188,734 and 5,171,445, the disclosure of which are hereby incorporated by reference, disclose asymmetric membranes and method for their production. Each of the Wrasidlo and Zepf patents disclose integral, highly asymmetric, microporously skinned membranes, having high flow rates and excellent retention properties. The membranes are generally prepared through a modified "phase inversion" process using a metastable two-phase liquid dispersion of polymer in solvent/nonsolvent systems which is cast and subsequently quenched in a nonsolvent. The Zepf patent discloses an improvement on the Wrasidlo patent.
The phase inversion process generally proceeds through the steps of casting a solution or a mixture comprising a polymer, a solvent, and a nonsolvent into a film, tube or fiber and precipitating the polymer. The most common method of precipitating the polymer is quenching in a nonsolvent liquid, generally water.
The nonsolvent and its concentration in the casting mix produce a region of instability which causes rapid precipitation when quenched in the nonsolvent liquid. This rapid precipitation may form a microporous skin at the interface and consequently can result in a highly asymmetric membrane.
In the past, when membranes were cast from chemically modified sulfone polymers, such as sulfonated sulfones, much of the potential for controlled variation in membrane structure, such as high cross sectional asymmetry of the membrane, was lost. It has been difficult to cast a highly asymmetric membrane from a hydrophilic polymer by conventional methods.
There is therefore a need for a method for preparing hydrophilic membranes which have pores large enough for microfiltration rather than ultrafiltration. Further, there is a need for a method of preparing hydrophilic membranes containing a range of sulfonated polymers with consistent properties. There is also a need for a method of preparing such membranes in which the hydrophilic components are nonleachable in water so that the membranes do not become hydrophobic when the hydrophilic components are leached out by water. In addition, there is a need for a method of casting membranes from casting solutions containing polysulfones and polyethersulfones together with sulfonated polymers to produce asymmetric membranes with longer lifetimes than symmetric membranes. Finally, there is a need for a method of producing hydrophilic membranes having a wide range of pore distributions.